Exterior Gateway Protocols are utilized for inter-Autonomous System routing to exchange reachability information for a set of networks internal to a particular autonomous system to a neighboring autonomous system.
The area of inter-AS routing is a current topic of research inside the Internet Engineering Task Force. The Exterior Gateway Protocol (EGP) described in Section [Appendix F.1] has traditionally been the inter-AS protocol of choice, but is now historical. The Border Gateway Protocol (BGP) eliminates many of the restrictions and limitations of EGP, and is therefore growing rapidly in popularity. A router is not required to implement any inter-AS routing protocol. However, if a router does implement EGP it also MUST IMPLEMENT BGP. Although it was not designed as an exterior gateway protocol, RIP (described in Section [7.2.4]) is sometimes used for inter-AS routing.